1. Field
The following description relates to a power supply and an organic light emitting display device using the same.
2. Description of Related Art
Recently, various flat panel display devices having reduced weight and volume as compared to cathode ray tubes (CRTs) have been developed. Among these flat panel display devices are liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices, among others.
Among the flat panel display devices, the organic light emitting display device displays images using an organic light emitting diode (OLED) that generates light through the recombination of electrons and holes corresponding to a flow of current. In the organic light emitting diode OLED an emission layer is made of organic material.
The organic light emitting display device as described above has excellent color representation, a thin thickness, and other features, so that its market has been largely expanded to applications in PDAs, MP3 players, and other devices, along with cellular phones.
FIG. 1 is a circuit view of a pixel in a general organic light emitting display device. Referring to FIG. 1, the pixel includes a first transistor M1, a second transistor M2, a capacitor Cst, and an organic light emitting diode OLED.
The first transistor M1 generates a driving current, where its source is connected to a first pixel power supply ELVDD, its drain is connected to an anode electrode of the organic light emitting diode OLED, and its gate is connected to a first node N1. Therefore, the driving current flows from the source to the drain, corresponding to the voltage at the first node N1.
The second transistor M2 selectively transfers a data signal to the first transistor M1, where its source is connected to a data line Dm, its drain is connected to the first node N1, and its gate is connected to a scan line Sn. Therefore, the data signal from the data line Dm is transferred to the first node N1 corresponding to a scan signal transferred through the scan line Sn.
The capacitor Cst maintains the voltage of the data signal applied to the gate of the first transistor M1, where its first electrode is connected to the first pixel power supply ELVDD and its second electrode is connected to the first node N1.
The organic light emitting diode OLED emits light corresponding to the driving current, where an emission layer is formed between the anode electrode and the cathode electrode to emit the light corresponding to the driving current. The anode electrode of the organic light emitting diode OLED is connected to the drain of the first transistor M1, and the cathode electrode is connected to a second pixel power supply ELVSS.
In the pixel as described above, the current flowing to the organic light emitting diode OLED is represented by the following equation 1.
                              I          oled                =                                            β              2                        ⁢                                          (                                  Vgs                  -                  Vth                                )                            2                                =                                    β              2                        ⁢                                          (                                  ELVDD                  -                  Vdata                  -                  Vth                                )                            2                                                          Equation        ⁢                                  ⁢        1            
Here, Ioled represents the current flowing to the organic light emitting diode OLED, Vgs represents the voltage between the gate and source of the first transistor M1, Vth represents the threshold voltage of the first transistor M1, ELVDD represents the voltage of the first pixel power supply, and β represents a constant.
In other words, a magnitude of the driving current flowing to the organic light emitting diode OLED changes when the voltage of the first pixel power supply ELVDD fluctuates.
The pixel receives the first pixel power ELVDD and the second pixel power ELVSS from a power supply (not shown). The power supply boosts input voltage from the outside to generate the first pixel power ELVDD and inverts the input voltage to generate the second pixel power ELVSS.
At this time, reviewing equation 1, the current flowing to the organic light emitting diode OLED flows corresponding to the voltage of the first pixel power supply ELVDD. If the voltage of the first pixel power supply ELVDD is varied, the amount of current flowing to the organic light emitting diode OLED is also varied, causing non-uniform brightness or luminance.
The power supply receives voltage from a battery or other constant voltage source to generate the first pixel power supply ELVDD and the second pixel power supply ELVSS, where there may be differences in the voltage level between the voltage supplied from the battery and the voltage supplied from other constant voltage source. In this case, if the voltage level of the first pixel power ELVDD generated when the voltage is supplied from the battery is different from the voltage level of the first pixel power ELVDD generated when the voltage is supplied from another constant voltage source, differences in brightness may occur.